Fixing comment downvote after an upvote.

[dessalines]

• The culprit was an incorrect on_conflict trigger, which needed to be converted to a MERGE to handle the multiple unique keys: (person_id, post_id) and (person_id, comment_id)
• Fixes Remove unnecessary PostActions::remove_like call #6103

[dullbananas]

Be careful with merge, because it handles concurrency differently, and a transaction is not necessarily enough to make it work properly. I'm not totally sure if this merge statement is fine.

I recommend adding a check that throws an error (which undoes the transaction) if the merge statement did not affect the correct number of rows, to avoid leaving inconsistent state in the database.

[dessalines]

Turns out I didn't need merge, just needed to change post_id to thing_id